As electrical insulation, especially for aircraft frame wire insulation, materials such as polytetrafluoroethylene (hereinafter "PTFE"), copolymer of ethylene and tetrafluoroethylene (hereinafter "ETFE"), crosslinked-ETFE, polyimide, polyester, etc. have often been used through extrusion or tape-wrap processes. Moreover, glass cloth impregnated with PTFE or continuously porous PTFE sheet impregnated in the surface portion with meltable resins have been used as substrates for printed circuit boards for high frequency applications.
However, the materials mentioned above do not have the performance characteristics required for electrical air-frame wire insulation such as light weight, high mechanical strength, cut-through resistance, heat resistance, and chemical resistance altogether at one time. That is, PTFE is not mechanically strong; ETFE and crosslinked ETFE have high specific gravity; polyimide is stiff and undergoes hydrolysis; and polyester is not highly heat resistant. Moreover, these materials, except for PTFE, have a dielectric constant exceeding 2.1. Thus, none are ideally suited for the dielectric insulation material of coaxial cables for high speed signal transmission.
Continuously porous PTFE, such as expanded PTFE, excels in that it has a dielectric constant far less than 2.1, and low density, however, it is not as mechanically strong as desired for air-frame electrical wiring. Thus it does not meet all the requirements together.
PTFE-impregnated glass cloth has an increased dielectric constant, so that its use as a substrate material is limited in terms of electrical frequency. Continuously porous PTFE substrate material impregnated in the surface region with meltable polymer resins has a porous intermediate region or area where the resin is not present so the dielectric constant remains low, thus it can be used as a high frequency substrate material or electrical insulation for high speed signal transmission wiring. However, due to the porous region, this material can easily be deformed by mechanical stress, causing the electrical properties to change. Thus, printed circuit boards or insulated wiring using this material need to be protected from outside stress, and their use is therefore limited.
Polyetheretherketone (PEEK) is also known as an insulation material for electric wiring. PEEK has excellent properties such as mechanical strength, heat resistance, substantial chemical resistance (although not as chemically inert as PTFE), and is highly resistant to damage by scratching and abrasion. However, it is stiffer than many of the polymers above, and can be difficult to extrude in thin layers. Consequently, when PEEK is extrusion-coated on wire, difficulties in obtaining thin layers or maintaining uniform wall thickness may lead to unwanted stiffness and weight, and to loss of flexibility and handleability.